1. Field of the Invention
The present invention relates to a technology of a lithium-ion capacitor, and more particularly, to a technology which is well adaptable to a negative electrode material of a lithium-ion capacitor.
2. Description of the Related Art
The following technologies were investigated by the inventor of the present patent application and are briefly referred to below.
In the recent situation where the environmental issue, particularly the vehicle-exhaust gas emission, is widely talked about, efforts are made of developing environment-friendly electric vehicles and the like. In the electric vehicle development, the strong development effort is focused on the electricity storage device to be used as a power source. Many types of electricity storage devices have been proposed for replacement of the conventional lead battery, and the papers discussing electrolytes, electrode materials and the like are increasing in number.
JP-A-2005-203130, for example, discloses a technology to lessen the deterioration of the input and output characteristics at the time of charging and discharging by using a graphite material of which an average particle size is within a range from 3 to 15 μm for a negative electrode material of a lithium-ion secondary battery.
JP-A-10-226506 describes that, for example, the cycle life and the charging and discharging efficiency of the battery are improved by using graphite powder of which an average particle size D50 by weight is 5 to 35 μm, which is obtained by the laser analysis scattering method with a maximum particle size of 75 μm, for a negative electrode material of a lithium secondary battery.
JP-A-06-52860 discloses a technology which suppresses the self-discharge, which results from a reaction of graphite with an electrolyte, to practically negligible levels in such a manner that graphite powder having a particle size of less than 10 μm is not used as a negative electrode material which can absorb and desorb lithium in a lithium secondary battery.
JP-A-08-31420 discloses that in a lithium battery, its mixing property with a binder at the time of electrode formation and decomposition of an electrolyte on the electrode surface can be suppressed in such a manner that a particle size of mesophase small spheres (meso carbon micro beads), which are produced through heat treatment of pitches, is specified by D10, D50 and D90.
The lithium-ion secondary battery or the like is advantageous in that it has high energy density, but has still technical problems. For example, its safety is still uncertain and its cycle life is not long. To cope with this, there has proposed an electric double layer capacitor as a novel electricity storage device. The electric double layer capacitor has a smaller discharging capacity per charge than that of a battery, but it is excellent in instantaneous charging/discharging characteristic and has such excellent output characteristics that it endures several tens of thousands cycles of charging/discharging operations.
Particularly, as disclosed in International Publication WO2003/003395, a novel lithium-ion capacitor is attracting attention in those fields. In the lithium-ion capacitor, a negative electrode is pre-doped with lithium ions to thereby keep a potential of the negative electrode low, and to secure the safety comparable with that of the electric double layer capacitor, high output characteristics, and high energy density.
As described above, various types of next generation electricity storage devices, which will come after the lead battery, have been proposed. However, the technologies of those proposed ones still involve many problems to be solved.
The lithium-ion capacitor in which the inventor has engaged for its development is also still unsatisfactory in energy density, output density, cycle durability and the like.